Aromatic conversion processes, and in particular alkylation of aromatic substrates, are involved in the production of a wide variety of petrochemical products. For example, alkyl substituted aromatics such as ethyl benzene and ethyl toluene are employed as intermediates which are converted to important styrene and vinyl toluene monomers useful in the production of a variety of styrene polymers. At present many alkylation processes include processing steps in which the aromatic substrates are contacted under alkylation conditions in the presence of catalyst materials. Both single and multiple catalyst bed processes are well known in the art.
Catalyst properties that are important in the conversion process include the selectivity of the catalysts to the desired product and the activity of the catalyst both initially, i.e., when the catalyst is fresh, and as a function of time. The selectivity of the catalyst is characterized as the concentration of the desired product in the total product, expressed as weight percent or mole percent, and the conversion activity of the catalyst is characterized as the amount of the desired product expressed as a mole percent of the stoichiometrically limiting reactant. Catalysts in hydrocarbon conversion processes are subject to being "poisoned" with use due to various factors such as coke accumulation on the catalyst, or the presence of catalyst poisons such as sulfur and other impurities in the feedstream.
Among the catalysts which may be employed in the alkylation of aromatic compounds are the shape-selective molecular sieves which include the zeolites. Zeolites are crystalline alumino-silicates which have ion exchange capacities. Thus, U.S. Pat. No. 4,016,218 to Haag et al discloses the alkylation of aromatic compounds by a process in which the aromatic charge stock and the olefinic alkylating agent are brought into contact with an alumino-silicate zeolite. The zeolite has a constraint index of one to twelve and more than 50% of the cationic sites on the catalyst are occupied by hydrogen ions. Specifically disclosed for use in the Haag et al process are the zeolites identified as ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38. Another alkylation process employing zeolite catalysts is disclosed in U.S. Pat. No. 4,365,104 to Kaeding. In this case, the zeolite catalyst is treated with a sulfur based agent, preferably after prior treatment with phosphorus and magnesium compounds, in order to increase the para-selectivity of the catalyst. Kaeding discloses that the zeolite catalyst may be treated with a highly oxidized compound such as sulfur dioxide or a highly reduced compound such as hydrogen sulfide. The catalysts disclosed in Kaeding include those disclosed in the aforementioned Haag patent and in addition a "highly siliceous" zeolite identified as ZSM-48. This catalyst, while characterized as having a silica to alumina ratio of up to infinity, is disclosed as having an ion exchange capacity and thus appears to be a true zeolite rather than a silicalite of the type described below.
Crystalline silicalites are another class of molecular sieves useful as catalysts. The silicalites, while having topological configurations similar to those of the zeolites, do not include the tetrahedral alumina structure characteristic of zeolites and are not ion exchangers. Silicalites have also been employed to advantage in the alkylation of aromatic compounds. Thus, U.S. Pat. No. 4,387,260 to Watson et al discloses an aromatic alkylation procedure in which steam is fed to the catalytic reactor along with the aromatic feed stock and the alkylating agent. The steam functions to maintain the alkylation activity of the catalyst and to increase the selectivity of the process for desired alkyl aromatics.
Heretofore, the zeolite catalyst systems set forth in the prior art have not been disclosed as useful in the alkylation of aromatic feed stocks containing a significant quantity of sulfur as a contaminant. In fact, the commercial specifications for benzene, a common feedstock used in aromatic alkylation procedures, suggest that such catalysts are not sulfur tolerant. Thus, the specification Benzene-535 ASTM 2359-66T requires that the benzene be free of sulfur compounds such as H.sub.2 S and SO.sub.2 and contain no more than one part per million (PPM) thiophene. Other specifications require a maximum total sulfur concentration of 2 ppm. Similarly, the aforementioned patent to Kaeding, while disclosing that hydrogen sulfide or sulfur dioxide will increase the para selectivity of a ZSM-5 type zeolite catalyst, specifies that the catalyst be contacted with the treating agent prior to the alkylation process and then calcined prior to the alkylation process. The pretreatment preferably takes place in an anhydrous environment.